The invention relates to a multilayer body, a method for the production thereof, a security element with such a multilayer body and a security document with a security element of this type.
Holograms are used as security elements to protect security documents such as banknotes, money substitutes, credit cards, passports or ID documents, as well as for product security. In the case of mass-produced goods surface holograms are often used which can achieve interesting optically variable effects, for example movement effects, and are characterized by a high luminous intensity.
In contrast to surface holograms, volume holograms, also called white-light holograms, are based on light diffraction at the Bragg planes inside a transparent layer which thereby has local differences in refractive index.
A surface relief can be used as the master to produce a multilayer body which contains a volume hologram. The front of the master is brought, directly or with a transparent optical medium interposed, into contact with a photosensitive layer of the multilayer body in which the volume hologram is to be drawn. The master is then illuminated with coherent light through the photosensitive layer, wherein by overlaying the light irradiated onto the master and the light diffracted by the master an interference pattern forms which is recorded in the photosensitive layer as a volume hologram. The volume hologram thus introduced into the photosensitive layer is then fixed after curing of the photosensitive layer. Through a specific design of the master, two or more separate items of image information can here be inscribed into the photosensitive layer.
Volume holograms generally have a high transparency and should therefore be applied to dark backgrounds in order to guarantee a good visibility or a high brilliance of the optical effects. This often makes expensive preparation work necessary on the substrates to which such security elements are to be applied.